The present invention relates to a copier, facsimile transceiver, laser printer or similar image forming apparatus having an image carrier and, more particularly, to a developing unit incoporated in such an apparatus for developing a latent image electrostatically formed on the image carrier by a developer or a toner deposited on a developer carrier.
A developing apparatus of the type described includes a toner hopper storing a developer or a toner. The developer or the toner is agitated by, for example, an agitator, charged by a charging roller, and then deposited on the surface of a developer carrier implemented as a developing roller. The developing roller is rotated to transport the developer or the toner to a developing region where it faces the surface of a photoconductive element, or image carrier, while a blade or similar regulating member regulates the thickness of the toner on the developing roller. On reaching the developing region, the developer or the toner develops an electrostatic latent image formed on the photoconductive element to convert it to a toner image. After image transfer, the developer or the toner remaining on the photoconductive element is collected in the toner hopper as the developing roller rotates.
The above-described construction has a problem that while the developing roller is in rotation, the toner enters the gap between the roller and each side wall of the developing unit. This part of the toner rubs against and sticks fast to the developing roller and side walls to obstruct the rotation of the roller, resulting in defective images. In the case of a developing roller implemented as a developing sleeve (mainly made of aluminum) having a magnet roller thereinside, the sleeve traverses the magnetic fields of the magnet roller to generate eddy currents and thereby increases the rotation torque of the roller or heats the roller. The heat causes the toner stored in the developing unit to melt and form masses. The toner is apt to drop from axially opposite ends of the developing roller onto a paper or similar recording medium, degrading image quality. The fact that the developing roller needs drive means and has to have a cylindrical configuration critically limits the layout of the developing unit. Moreover, when the blade for regulating the thickness of toner is held in contact with the developing roller, a desirably thin toner layer and, therefore, attractive images are not achievable unless the blade is positioned with high accuracy. Especially, the blade easily wears and has to be replaced at a predetermined interval. In addition, the toner is apt to stick fast at a position where the blade and developing roller contact, also degrading the image quality.
On the other hand, a bicolor, tricolor, full color or similar multicolor copier is provided with a plurality of developing units each accommodating a toner of particular color. Each of the developing units is moved from an inoperative position remote from a photoconductive element to an operative position close thereto only at the time of development. This is to prevent the different colors from being mixed on a paper. This kind of scheme, however, needs a mechanism for moving the developing units toward and away from the photoconductive element, resulting in a complicated construction. Further, it is difficult to maintain the gap between a developing roller received in each developing unit and the photoconductive element at a developing position with relatively high accuracy.
In an image forming apparatus of the type having a removable developing unit, it is likely that a toner remaining on part of the surface of a developing roller exposed to the outside of the developing unit is scattered around when the developing unit is pulled out of the apparatus.